1. Technical Field of the Invention
The present invention relates in general to the mobile telecommunications field and, in particular, to a method and system for processing multiple random access calls in a Code Division Multiple Access (CDMA) or Wideband CDMA (WCDMA) system.
2. Description of Related Art
For the next generation mobile communication systems, such as the IMT-2000 and Universal Mobile Telecommunications System (UMTS), Direct Sequence-CDMA (DS-CDMA) approaches have been proposed for use in the United States, Europe and Japan. In this regard, a similar WCDMA system is being considered for use in both Europe and Japan, but a somewhat different DS-CDMA concept is being considered for use in the United States.
These next generation systems will be required to provide a broad selection of telecommunications services including digital voice, video and data in packet and channel circuit-switched modes. As a result, the number of calls being made is expected to increase significantly, which will result in much higher traffic density on random access channels (RACHs). Unfortunately, this higher traffic density will also result in increased collisions and access failures. Consequently, the new generation of mobile communication systems will have to use much faster and flexible random access procedures, in order to increase their access success rates and reduce their access request processing times. In other words, there will be a high demand for much faster and more efficient access in those systems due to the expected substantial increase in packet-switched traffic.
The proposed WCDMA approach includes two different ways to transmit packets, on a common channel and a dedicated channel. However, there will be a high demand for faster and more efficient random access using either transmission scheme. For example, commonly-assigned U.S. patent applications Ser. Nos. 08/733,501 and 08/847,655, and U.S. Provisional Application Serial No. 60/063,024 describe such a random access approach, which can be used for a packet-based service where a mobile station (MS) can transmit packets on a common channel and a dedicated channel. For the common channel case, the packets are included in the random access requests being transmitted. For the dedicated channel case, the random access requests being transmitted include requests for a dedicated channel on which to transmit the associated packets.
The above-described patent applications disclose a Slotted-ALOHA (S-ALOHA) random access approach. Using this approach, a common transmission medium can be shared by a plurality of users. Essentially, the transmission medium is divided into a plurality of access slots, which are characterized by a time offset relative to the received frame boundary. Each user (MS) randomly selects an access slot and then transmits its message information in that access slot. However, a shortcoming of this approach is that the access slots are not allocated to the users, which can result in collisions between the different users"" transmissions.
For example, using the S-ALOHA random access approach in the above-described patent applications, a MS generates and transmits a random access request. A diagram that illustrates a frame structure for such a random access request is shown in FIG. 1. The frame structure shown is used in the first two of the above-described patent applications. As shown, the random access request comprises a preamble and a data field portion. The preamble part is used primarily as a ringing function. The data portion includes the request and/or the data packet. In order to reduce the risk of collisions for requests from different MSs that choose the same access slot, the preamble for each MS""s request contains a unique signature (bit) pattern. The MSs randomly select the signature patterns used (preferably from a limited set of signature patterns), which further reduces the risk of collisions.
The following procedure is typically used in an S-ALOHA random access system. First, an MS is synchronized to a base station. The MS xe2x80x9clistensxe2x80x9d to a broadcast channel over which, for example, the network broadcasts random access codes, broadcast channel transmit power level, and the interference signal level measured at that base station. Next, the MS estimates the downlink path loss, and together with the knowledge of the base station interference signal level and the transmit power level, estimates a transmit power level to use. The MS then selects an access slot and signature pattern, and transmits its random access request on the selected access slot and with the selected signature pattern. The MS awaits an acknowledgment to the access request from the base station. If the MS does not receive an acknowledgment from the base station within a predetermined (time-out) period, the MS selects a new access slot and signature pattern, and transmits a new random access request.
Referring to FIG. 1, the preamble portion is modulated with different signature patterns, and spread with a base station-unique spreading code. The signature patterns are used for separating different simultaneous random access requests, and also to determine which spreading/scrambling code to use on the data portion of the requests. Consequently, as mentioned earlier, the requests from two different MSs that use the same access slot but with different signature patterns can be separated. Also, pilot symbols can be inserted into the data portion of the request, in order to facilitate coherent detection. The preamble portion of the request can also be used for coherent detection, but if the data portion is relatively long, the channel estimate has to be updated accordingly.
FIG. 2 illustrates the frame structure of the random access request described in the third of the above-described patent applications. Using the frame structure shown, the data portion is transmitted on the I branch of the channel, and the preamble/pilot is transmitted on the Q branch. This frame structure is used in order to make the random access channel compatible with the other dedicated uplink channels used, which for the WCDMA approach is I/Q multiplexed. In any event, it does not matter whether the data and pilot symbols are time-multiplexed, I/Q multiplexed, or code-multiplexed (which can be performed among other methods by complex scrambling an I/Q multiplexed signal).
A frame is divided into a number of time slots on the dedicated data channel according to the power control command rate. These slots are denoted frame slots. In the proposed WCDMA systems, there are 16 of these frame slots per frame. In a random access scheme, a frame is also sub-divided into a number of access slots, but the purpose is to increase the throughput efficiency of the random access process. An access slot defines a period in which an MS can start its transmission of a random access request. Using the random access approach in the first two of the above-described patent applications, the random access requests can, for example, be transmitted in consecutive access slots as shown in FIG. 3.
The data portion of the random access requests shown in FIG. 3 is scrambled by a long code (same length as the data portion). Consequently, an access slot plus a guard time can be equal to N frame slots. Preferably, the preambles from different access slots should not overlap, because there would be too many preamble detectors required in the base station, and the interference (due to the same spreading codes being used) would be increased for the random access detection process. However, for the frame structure used in the third of the above-described patent applications, the throughput efficiency of the random access channel may be reduced, because longer preambles are being used and the preambles of different access requests in different access slots should not overlap.
The random access receiver in the base station is comprised of two sections, wherein one section detects the preamble, and the second section detects the data portion of the request. The section that detects the preamble includes a matched filter, which is matched to the spreading code used on the signatures. The modulation of the output signal of the matched filter is removed by multiplication with the expected signature symbols (remodulation), in order to separate random access requests from different MSs that have used different signatures.
When a random access request is registered in the preamble detector section of the base station receiver, a plurality of RAKE fingers are allocated in order to detect the data portion of that request. Also, the preamble detector section couples the frame timing of the data portion of the request to the RAKE receiver, along with the spreading code used on the data portion, and an initial estimate of the channel response. The RAKE receiver detects the data from the data portion, and the base station processes the data and responds to that MS""s random access request.
A problem with the above-mentioned application""s approach is that the random access channel used is not compatible with the other uplink channels used in the proposed WCDMA approach. Consequently, new hardware needs to be developed for the data portion of the random access channel.
A problem with the third above-described application""s approach is that although it avoids the uplink channel compatibility problem, it requires a significant amount of additional buffering. Another problem with this approach is that the random access request message processing rate will be reduced, because the preambles from different access slots should not overlap, and the preambles in this approach are relatively long.
A problem with the third random access approach (described in the third application), which does not exist for the other approaches, is that if the data portion of the request is longer than one access slot, then an ambiguity in detection of the frame timing may exist. In that case, the pilot symbol in each access slot may carry a signature which is the same in each access slot, or the signature may be changed from access slot to access slot. As such, there can be numerous times during a data transmission when a signature is detected. However, the base station receives one timing signal per access slot, and therefore, there can be a problem in determining the exact frame timing. Although this problem can be solved with existing means, such a solution is rather complicated.
An additional problem with this approach is that during the random access detection process, the complete access slot has to be buffered for subsequent data detection until the random access request has been detected by decoding the simultaneously transmitted signature pattern. This step takes one access slot to complete and thus requires maximum buffering of one complete access slot.
Additional buffering is also required during the data portion detection used in the other two approaches (as well as in the method of the present invention), because channel estimation has to be performed based on a continuously transmitted pilot channel (approach three above), or periodically inserted pilot symbols (approach one above). In other words, the channel estimates have to be provided in parallel with each received data symbol. The buffering needed is only for as long as it takes to calculate a channel estimate related (i.e., transmitted during the same time) to a data symbol.
In accordance with a preferred embodiment of the present invention, an uplink common physical channel (random access channel) frame structure is provided with a separate preamble and data portion. The preamble is used by the base station to detect that a MS is attempting the random access request. The data portion of the channel includes user data, and pilot symbols that provide energy for channel estimation during reception of the data portion. A guard interval is preferably inserted in the preamble portion of the frame, which enables some data detection to occur before the actual data detection process is started. Consequently, the buffering of data can be minimized.
An important technical advantage of the present invention is that the frame structure on the common physical uplink channel is compatible with the frame structure on the dedicated physical uplink channel.
Another important technical advantage of the present invention is that each portion of the random access request has to fulfill only one function and can thus be optimally designed for that respective task.
Still another important technical advantage of the present invention is that the same type of code allocation scheme can be used for both the data portion of the random access request and the dedicated uplink channels.
Yet another important technical advantage of the present invention is that all necessary post-processing, such as for example, signature decoding, can be accomplished during a guard period. Consequently, the hardware design for random access request detection can be simplified, and the random access request processing delay can be minimized.
Still another important technical advantage of the present invention is that the same receiver hardware can be used for decoding both the data portion of the common physical uplink channel and the conventional dedicated physical uplink channel, which unifies the hardware design and lowers the hardware costs.
Yet another important technical advantage of the present invention is that a pool of RAKE receivers or RAKE fingers can be assigned or shared for both the common physical channel (random access data packet) and dedicated physical channel (traffic channel), which minimizes the amount of hardware required.
Still another important technical advantage of the present invention is that the buffer size requirements can be minimized, because the functions of the preamble and data portion of the random access request are separated.
Still another important technical advantage of the present invention is that the random access request rate can be increased in comparison with other random access approaches. In particular, the random access request rate for the third of the above-described random access approaches would be lower than that for the present invention for the same amount of hardware used.
Yet another important technical advantage of the present invention is that a capability for transmitting the random access messages at different rates can be achieved in a very flexible way.